1. Field of the Invention
The present invention relates to an optical element which homogenizes energy distribution of a beam spot on an irradiated surface in a specific region, and a light irradiation apparatus using the optical element. Moreover, the invention relates to a manufacturing method of a semiconductor device in which a crystalline semiconductor film formed using the light irradiation apparatus is used.
2. Description of the Related Art
In recent years, technique for manufacturing a thin film transistor (hereinafter, referred to as a TFT) over a substrate has been drastically advanced, and the application and development thereof to an active matrix display device has been promoted. In particular, a TFT in which a polycrystalline semiconductor film is used has higher electron field-effect mobility (also referred to as mobility) than a conventional TFT in which a non-single crystal semiconductor film is used; therefore, high speed operation can be conducted. Hence, the control of a pixel which is conventionally conducted in a driver circuit provided outside a substrate has been tried to conduct in a driver circuit formed on the same substrate as a pixel.
Meanwhile, as for a substrate used for a semiconductor device, a glass substrate is considered to be more promising substrate than a single crystal semiconductor substrate in terms of cost. The glass substrate has an advantage that an inexpensive and large-sized substrate can be easily manufactured compared with a synthesized quartz substrate, but also has a disadvantage that a melting point is lower than a synthesized quartz substrate. However, in the case where laser annealing is conducted using a laser beam to a semiconductor film formed on a glass substrate, it is possible to make only a temperature of the semiconductor film high, and accordingly, crystallization, flattening, or surface modification of the semiconductor film can be conducted by giving little thermal damage to the glass substrate. In addition, throughput is remarkably high compared with a heating means using an electrically-heated oven. Therefore, technique for conducting laser annealing to a non-single crystal semiconductor film (a non-single crystal semiconductor, namely a semiconductor having crystallinity such as polycrystal or microcrystal but not a single crystal, or an amorphous semiconductor) formed on a glass substrate is widely studied. Laser annealing herein used indicates technique for crystallizing a damaged layer or an amorphous layer formed on a semiconductor substrate or a semiconductor film or technique for crystallizing a non-single crystal semiconductor film formed on a substrate, and also includes technique which is applied to flattening or surface modification of a semiconductor substrate or a semiconductor film.
A crystalline semiconductor film manufactured by laser annealing has high mobility; therefore, the crystalline semiconductor film has been actively used for an active layer or the like of a TFT for a driver circuit included in an active matrix display device.
As for a laser beam used in the laser annealing, a laser beam oscillated from an excimer laser is frequently used. The excimer laser has advantages that it has high output and it can conduct repeated irradiation with high frequency. Moreover, the laser beam oscillated from the excimer laser has an advantage that absorption coefficient to a silicon film which is often employed as a semiconductor film is high. As a method for laser annealing, there is an irradiation method by which the shape of a beam spot on an irradiated surface is reformed by an optical system to be a rectangular shape having a fixed region, and the irradiated position of the laser beam is moved relatively to the irradiated surface. This method has high productivity and is industrially superior. In this specification, a beam to have a rectangular shape in its beam spot on the irradiated surface is referred to as a rectangular beam, and irradiation to an object to be irradiated using the rectangular beam is referred to as area irradiation. It is to be noted that, in this specification, the rectangular beam is acceptable as long as the shape of the beam spot on the irradiated surface is almost a rectangular shape. Thus, four interior angles of the rectangular beam are not required to be strictly a right angle, respectively, and may be rounded to some extent. In addition, a rectangular beam which has especially high aspect ratio (specifically, aspect ratio of 10 or more, preferably 100 to 10,000) is a linear beam, which is distinguished from the rectangular beam in this specification. In this specification, irradiation of the linear beam to an object to be irradiated by relatively moving the linear beam in a direction orthogonal to a direction in which the beam width of the linear beam is longer, is referred to as line irradiation.
The intensity distribution of the laser beam oscillated from a light source is generally Gaussian distribution, and in order to conduct homogeneous laser annealing, it is necessary to homogenize the intensity distribution of the laser beam. In recent years, a method for homogenizing intensity distribution of a laser beam, by which the laser beam is divided into a predetermined direction using a cylindrical lens array and each of the divided laser beam is overlapped in the same face, is frequently employed as a method for homogenizing intensity distribution of a laser beam. By using the beam formed as described above, laser annealing of a semiconductor film formed on a large substrate comes to be able to be conducted more effectively.
However, in the case of using a cylindrical lens array, processing precision of individual cylindrical lens becomes a problem. A cylindrical lens array is a line of a plurality of cylindrical lenses; however, it is impossible to completely uniform a radius of curvature or profile irregularity of each cylindrical lens. Therefore, individual laser beam divided by the cylindrical lens array cannot overlap to be conformed completely on the irradiated surface, and accordingly, a region where intensity distribution is reduced is generated in a beam to be formed. This can be a problem in conducting laser annealing to a semiconductor film. In the case where a TFT is manufactured using a semiconductor film to which laser annealing is conducted by a beam with inhomogeneous intensity distribution described above, and further, a liquid crystal or an organic EL display is manufactured using the TFT, fringe or color unevenness is generated on a display in some cases.
Further, a conventional light irradiation apparatus does not have a structure which can form a beam spot with a desired size in accordance with the size of a region to be irradiated. Hence, in spite of the size of the region to be irradiated, a processing such as laser annealing is conducted using a laser beam with a fixed size, which takes a fixed processing time regardless of the size of the region to be irradiated.
A photolithographic apparatus capable of changing the dimension of an optical waveguide is disclosed in Patent Document 1 (PCT Publication No. WO 97/38356). An optical waveguide shown in Patent Document 1 has a feature of having inner walls movable with respect to each other; however, the inner walls which form the optical waveguide move only in a direction orthogonal to a longitudinal direction of the optical waveguide (width direction of a static irradiation field). In other words, as shown in FIG. 5a or 5b of the Patent Document 1, the dimension itself in the longitudinal direction is not changed though the dimension of the direction orthogonal to the longitudinal direction is changed before and after the movement of the inner walls of the optical waveguide. Therefore, the shape of the optical waveguide is changed before and after the movement of the inner walls of the optical waveguide.